1. Field of the Invention
This invention relates to electrical connections for electrical conductors and to a method for making the same. In particular, the invention relates to an aluminum-to-copper transition member for electrically connecting an aluminum wound motor, transformer or the like into an electrical circuit via a copper connection.
2. Description of the Prior Art
Reliability requirements for electrical connections are becoming increasingly more severe because of additional demands on temperature stability and structural strength of electrically interconnected members. For example, industrial motor requirements include both resistance to severe mechanical stress as well as sustained operation at an elevated temperature range. These motors are designed to operate at a steady-state temperature winding temperature of 125.degree. C. and to experience a temperature rise to about 180.degree. C. on full-voltage reversal and to about 300.degree. C. during 20 second stall conditions. Additionally, the electrical wires of the circuit must be capable of functioning at current densities of about 1000 amperes per square inch and higher. In particular, where the wire elements or electrical wires require an electrical connection between an aluminum electrical conductor and a copper electrical conductor, mechanical stress and heat of joining the two together physically are very important. Interconnection of the two conductors is traditionally made by crimping, soldering, brazing and like methods to provide initially a good solid electrical connection between the two different type conductors. With extreme vibration, high current densities and sustained operation at elevated temperatures, electrical connections made in this manner are unreliable. Failures occur because of mechanical disengagement or burnoff of the connection.
In an aluminum wound industrial motor, a crimp connection depends, for reliability, upon the maintaining of a high interface pressure between the aluminum wires and a connector body which is typically tin-plated copper or copper alloy. In industrial motors wherein the windings are of copper, there is no expansion mismatch with the connector body and the steady-state motor temperature is below that at which significant stress-relaxation would occur. Therefore, a high interference pressure necessary for low resistance joints can be maintained.
However, aluminum has an expansion coefficient about 50% higher than that of the copper connector body. Since the residual compressive stress on the aluminum wires will be of yield-point magnitude after completion of the crimp at room temperature, exposure to high operating temperature ranges can be expected not to result in a further increase in interface pressure because of expansion mismatch. Any tendency for an increase in interface pressure will merely result in plastic flow of the aluminum wire since the aluminum is already at its yield strength. Plastic flow of the aluminum wire also occurs because the yield strength and the creep strength of aluminum are lower at the higher temperatures than for aluminum at room temperature. The resulting plastic flow of the aluminum wires which occurs to relieve the interface stress is not reversible upon cooling to lower temperatures. The interface pressure is now less than the initial interface pressure. This change of interface pressure which occurs during each thermal cycle results in increasing the resistance of the interface. Consequently, the wires when carrying electrical current generate increasing amounts of heat at the connector and the thermal cycling of the wires results in increasing deterioration of the wire conductors and ultimate failure of the same.
Aluminum wire, when freshly shaved to achieve good electrical conductivity characteristics, forms an oxide film thereon almost immediately upon exposure to air The electrical resistance of the aluminum oxide is sufficient to prevent achieving low resistance electrical contacts without sufficient plastic flow of the aluminum wire. Plastic flow of the wire is necessary to fracture the oxide film to allow extrusion of the freshly exposed oxide-free aluminum into the cracks of the oxide layer thereby making a low resistance electrical contact with the conductors connected thereto. Progressive relaxation of interface pressure during thermal cycling can result in gradual formation of oxide around the periphery of the area of contact of the wire and the connector. The end result is a reduction in the effective area for electrical current transfer therebetween thereby increasing the current density and the thermal energy generated therein at the remaining areas of "clean" metal contact.
Many prior art electrical connections rely on crimp connectors of the "insulation-piercing" type. This enables one to avoid the necessity and cost of having to remove electrical insulation from the ends of wires to be joined together. The connectors may be of several types. One type has serrations on the inner surface of the connector which pierces through the electrical insulation to provide a metal-to-metal electrical contact between the wire and the connector. Another type of connector has an internal perforated screen of electrically conductive metal, such, for example, as brass, wherein the cylindrical surface face of the end portion of the aluminum wire is extruded through the screen to make a metal-to-metal electrical contact. A cap of electrical insulation remains as the top of the extruded button so that a metal-to-metal contact does not occur directly between the wire and the connector body.
The combinations of wire conductors which can be electrically joined together in a connector are innumerable. Wire conductors may be of a solid or a stranded conductor configuration. Each of the wires may be of a different size whether it be an individual conductor or a stranded design. Statistically, a better quality electrical contact is achieved between some wires of a group than with the remaining wires of the group. Large diameter wire conductors are deformed more than small diameter wires in a crimp connection because the external indentation of the crimp connector is fixed. The interface pressure between the connector and individual wires may differ considerably. Solid and stranded copper wires have a different yield and flow behavior than do aluminum wires. The interface pressures therebetween are different from that of aluminum wires.
In abandoned U.S. patent application Ser. No. 508,746, filed on Sept. 24, 1974 (now refiled as Continuation-in-Part Application Ser. No. 10,313, filed Feb. 7, 1979), which is a Continuation of U.S. patent application Ser. No. 400,012, filed on Sept. 24, 1973, and now abandoned, a cold-weld butt joint is described as being suitable for electrically conductive copper-to-aluminum transition members. However, automated production of these transition members is limited because of the method of joining required and the metal flash which must be removed. This cold weld butt joint method is also described in Product Engineering, December, 1974, Pages 19-22. Additionally, copper material, more costly at this time, is wasted since the conductors, or elements, must be of the same wire size physically for fabrication. However, since copper is more electrically conductive, it would be highly desirable to employ copper wire of smaller wire size material with aluminum of the same electrical conductivity but larger wire size physically.
Additionally, statistically, only some of the extruded aluminum wire will have a completely formed button, and the remainder will be only partially formed by the brass screen. A similar variation in contacting is achieved by the serrated portions of the other type connector. Both result in a variable quality of connection of given wires to a connector. This variability of connection quality inevitably leads to a condition in which certain wires of a group afford a lower resistance path between connectors and therefore, initially, carry a greater share of the total electrical current carried by the stranded cable. Consequently, these wires experience higher thermal gradients and earlier than anticipated degradation of the wire insulation resulting in premature motor failure. The "lazy wires", those which do not carry their share of the electrical load, do not begin to carry current until the increased resistance of the wires initially carrying the current approaches that of the "lazy wires".
An object of this invention is to provide electrical connections which substantially reduce the premature electrical failure of industrial electric motors, transformers, and the like.
Another object of this invention is to provide a reliable electrical connection between the windings of an aluminum wound electrical motor and copper wire connectors.
A further object of this invention is to provide a transition member embodying a cold weld lap joint for electrically connecting together aluminum and copper conductors.
A further object of this invention is to provide an electrical connection between aluminum and copper stranded wire conductors wherein all wire conductors in the stranded wire conductor carry substantially the same electrical current at all times.
A still further object of this invention is to provide a motor having all welded connections to eliminate the problem of "lazy wires" in the motor resulting in premature failures.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.